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Disk imaging is the process of making a bit-by-bit copy of a disk. Imaging (in more general terms) can apply to anything that can be considered as a bit-stream, e.g. a physical or logical volumes, network streams, etc.

The most straight-forward disk imaging method is reading a disk from start to end and writing the data to a Forensics image format.
This can be a time consuming process especially for disks with a large capacity.

Integrity

Often when creating a disk image a cryptographic hash is calculated of the entire disk. Commonly used cryptographic hashes are MD5, SHA1 and/or SHA256.

By recalculating the integrity hash at a later time, one can determine if the data in the disk image has been changed. This by itself provides no protection against intentional tampering, but can indicate that the data was altered, e.g. due to corruption. The integrity hash does not indicate where int he data the alteration has occurred. Therefore some image tools and/or formats provide for additional integrity checks like:

Smart imaging

Smart imaging is a combination of techniques to make the imaging process more intelligent.

Compressed storage

Deduplication

Selective imaging

Decryption while imaging

Compressed storage

A common technique to reduce the size of an image file is to compress the data. Where the compression method should be lossless.
On modern computers, with multiple cores, the compression can be done in parallel reducing the output without prolonging the imaging process.
Since the write speed of the target disk can be a bottleneck in imaging process, parallel compression can reduce the total time of the imaging process.
Guymager was one of the first imaging tools to implement the concept of multi-process compression for the Encase image file format. This technique is now used by various imaging tools including Tableau Imager (TIM)

Other techniques like storing the data sparse, using empty-block compression or pattern fill, can reduce the total time of the imaging process and the resulting size of new non-encrypted (0-byte filled) disks.

Deduplication

Deduplication is the process of determining and storing data that occurs more than once on-disk, only once in the image.
It is even possible to store the data once for a corpus of images using techniques like hash based imaging.

Selective imaging

Selective imaging is a technique to only make a copy of certain information on a disk like the $MFT on an NTFS volume with the necessary contextual information.

EnCase Logical Evidence Format (LEF) is an example of a selective image; although only file related contextual information is stored in the format by EnCase.

Decryption while imaging

Encrypted data is worst-case scenario for compression. Because the encryption process should be deterministic, a solution to reduce the size of an encrypted image is to store it non-encrypted and compressed and encrypt it again on-the-fly if required. Although this should be rare since the non-encrypted data is what undergoes analysis.